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DURHAM, NC (CBS) – Sure, some little girls aren’t all “sugar and spice and everything nice,” but could they be the cause of their parents’ divorce?

If research done in recent years is any indication, the answer might appear to be yes – studies show that marriages that produce firstborn daughters are statistically more likely to end in divorce – but the real question is, why?

Now, scientists from Duke University think they have part of the answer. Rather than starting their research at the birth of a child, they decided to begin their work at conception, since biologically, female embryos are “hardier” than male embryos.

“The [previous] findings have been interpreted as contemporary evidence of fathers’ son preference,” the researchers write. “Our study explores the potential role of another set of dynamics that may drive these patterns: namely, selection into live birth.”

In other words, while “Many have suggested that girls have a negative effect on the stability of their parents’ union,” Duke economist Amar Hamoudi, who co-authored the study published July 15 in Demography with Jenna Nobles, a University of Wisconsin-Madison sociologist, told The Durham News. “We are saying: ‘Not so fast.’ What about all those pregnancies that don’t end up in a live birth?”

Hamoudi and his team say that, since stress impacts fertility and the survival of an embryo, unstable marriages with frequent arguments could produce more female babies than male ones due to the “female survival advantage.” And if the marriage was already stressful enough before the child’s birth, the stress only increases with the arrival of a baby. Further research seems to back up their theory.

“Using data from the National Longitudinal Survey of Youth (NLSY79), we find that relationship conflict predicts the sex of children born after conflict was measured; conflict also predicts subsequent divorce,” the scientists say.

So, a firstborn baby girl might be the product of an unstable marriage in some cases rather than the cause of one.

“It’s not too surprising you should have a correlation between bad relationships and heart problems,” said Dr. Brian McDonough, medical editor at KYW Newsradio. “I’m not talking about someone whom you disagree with occasionally, but someone who really affects your day-to-day approach to life. These people are at greater risk for heart disease.”